Question: The scale on a map is 4cm : 6km. If two cities are 40cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 4cm on the map corresponds to an actual distance of 6km. A distance of 40cm on the map is the same as 10 $\cdot$ 4cm. The actual distance between the two cities is 10 $\cdot$ 6km, or 60km.